Amorphous silica-alumina is a well known composition that has uses in catalyzing hydrocarbon conversion processes and as a support for catalytic metals. One way of preparing such a composition is to separately form sols of aluminum and silicon, mix the sols together and then gel the sols by the oil drop method and dry the resultant particles to provide an amorphous silica-alumina composition. Such a procedure is described in U.S. Pat. No. 3,909,450. Other methods include coprecipitation as described in U.S. Pat. No. 3,274,124.
Applicants have found that amorphous silica-alumina compositions can be modified by reacting the amorphous silica-alumina composition with a fluorosilicate salt to remove some of the aluminum atoms in the amorphous silica-alumina and replace them with silicon atoms. This silicon enhanced amorphous silica-alumina (SEASAL) composition has increased acidity as shown by the fact that it has increased cracking activity. The increase in activity is considerably greater than if an amorphous silica-alumina composition were prepared with a greater silicon content. That is, the silicon that is added by the fluorosilicate salt is different from the silicon in the host amorphous silica-alumina. The SEASAL also contains fluoride.
Several references in the prior art deal with materials that have been treated with fluorosilicate salts. For example, U.S. Pat. No. 4,576,711 discloses reacting a Y-zeolite with an aqueous solution of ammonium hexafluorosilicate to remove some of the framework aluminum atoms and replace them with silicon. Similarly, U.S. Pat. No. 4,503,023 discloses the dealumination of zeolite LZ-210 with silicon substitution.
U.S. Pat. No. 4,753,910 discloses using a water soluble fluoride during or after the aluminum removal step in order to solubilize the aluminum fluoride which is produced during the aluminum removal (dealumination) step.
U.S. Pat. No. 4,711,770 discloses inserting silicon atoms into the crystal lattice of an aluminosilicate zeolite by contacting the zeolite with a fluoro-silicate salt at a pH of about 3 to 7 and at a rate to preserve at least 60% of the crystallinity of the zeolite. This patent also discloses materials which have defect sites in the framework.
U.S. Pat. No. 4,597,956 discloses a method of removing aluminum fluoride byproducts by contacting the aluminosilicate with a soluble aluminum compound such as aluminum sulfate.
U.S. Pat. Nos. 4,591,576 and 4,588,701 are companion cases which describe a catalyst containing a zeolite and an inorganic oxide matrix. The inorganic oxide matrix may be amorphous oxides such as silica, alumina, silica-alumina, silica-zirconia, silica-magnesia, alumina-boria, alumina-titania, etc. The catalyst is treated with a fluorosilicate salt to control the sodium oxide content of the catalyst. The patentees mention that separate treatment of the zeolite and inorganic oxide matrix with subsequent mixture is not desirable.
U.S. Pat. No. 4,806,513 discloses a support which is alumina having a surface coating of silica which can be treated with fluorine.
U.S. Pat. No. 2,483,131 discloses impregnating alumina or mixtures of alumina/silica/zirconia with silicon tetrafluoride.
Finally, U.S. Pat. No. 5,008,233 discloses treating a crystalline alumina with a fluorosilicate salt to give a silicon substituted alumina in which the silicon concentration is non-homogeneous.
Comparing this prior art with the instant invention, it is observed that none of the prior art discloses that it would be beneficial to remove aluminum atoms from an amorphous silica-alumina and replace them with silicon. The resultant SEASAL has at least 30% cracking activity (as defined herein) which is different from other amorphous silica-aluminas known in the prior art.